In embodiments, the present invention pertains to a polyethylene film with high tensile strength and high tensile energy to break, and to a method for manufacturing such a film from a specific ultra high molecular weight polyethylene. In this document ultra high molecular weight polyethylene will further be indicated as UHMWPE.
Methods for manufacturing high-strength high-modulus UHMWPE films are known in the art.
U.S. Pat. No. 5,756,660 describes polymerisation of UHMWPE over a specific catalyst, followed by compression moulding, rolling and stretching to form a polymer film. In example 1, a material is obtained with a modulus of 160 GPa and a strength of 2.8 GPa.
U.S. Pat. No. 5,106,555 describes a method for compression molding/stretching of UHMWPE.
U.S. Pat. No. 5,503,791 describes a polyethylene film manufactured by extruding a solution of a polyolefin in a first solvent, followed by cooling, the removal of the solvent and stretching of the film. A disadvantage of the thus obtained film is that it always contains a certain amount of residual solvent, which may detrimentally affect the properties of the film. In general the amount of solvent present in solution of gel-cast films is at least 100 ppm. Further, the recovery of solvent is highly uneconomic.
EP 292 074 describes a method for processing polyolefins with a Mw of more than 400.000, in particular between 500.000 and 10.000.000, a low entanglement density, and a Mw/Mn of less than 10, preferably less than 5 at a temperature below the melting point of the polymer using a process auxiliary agent. The polyolefin is first compressed in the presence of a processing aid, and then subjected to solid state processing. The processing aids mentioned in this reference are polymer solvents. In the Example 17 wt. % of decaline is used.
EP 269 151 describes a material having a tensile strength above 2.0 GPa, a modulus above 70 GPa. and a creep below 3*10−7 sec−1. This reference contains no information on the Mw/Mn ratio of the final product.
U.S. Pat. No. 5,106,558 describes a method for the continuous preparation of a polyethylene having high strength and high modulus which comprises the steps of mixing 100 parts by weight of an ultra-high molecular weight polyethylene powder having an intrinsic viscosity of 5-50 dl/g in decalin at 135° C. with 2-50 parts of a liquid organic compound having a boiling point higher than the melting point of polyethylene, feeding the resulting mixture between a pair of rollers, and subjecting the mixture to compression moulding and drawing. This reference contains no information on the Mw/Mn ratio of the final product.
U.S. Pat. No. 6,017,480 describes a process for producing polyolefin materials comprising the steps of preparing a molded article of UHMW polyolefin, stretching the article by more than 30-fold, contacting the article, and re-streching the article. This reference contains no information on the Mw/Mn ratio of the final product.
EP 2014445, which was filed before the priority date of the present application but published thereafter, describes a process for manufacturing polyethylene films. In embodiments, the present invention pertains to a UHMWPE film having a tensile strength of at least 2.0 GPa, a tensile energy to break of at least 30 J/g, an Mw of at least 500000 gram/mole, and a Mw/Mn ratio of at most 8. The film may be manufactured via a process which comprises subjecting a starting UHMWPE with a weight average molecular weight of at least 500000 gram/mole, an elastic modulus determined directly after melting at 160° C. of at most 0.9 MPa, and a Mw/Mn ratio of at most 8 to a compacting step and a stretching step under such conditions that at no point during the processing of the polymer its temperature is raised to a value above its equilibrium melting point.